The conventional technology of a fuel injection system will be described with respect to multiple injections (a multistage injection in which the fuel injection is divided into a plurality of injections) as shown in FIG. 9A. When a plurality of fuel injections are performed in a predetermined injection period in one cycle as shown in FIG. 9A, the second and subsequent injections are influenced by the previous injection (a pulsation generated in the piping which supplies fuel into an injector), resulting in a change comprising a delay in injection starting and a delay in injection termination.
This will be explained with reference to the detailed drawing of FIG. 9B.
When a driving signal for the injector such as a driving pulse is sent to an injector, and if the injection is not influenced by a pulsation, the injection rate starts increasing at a point in time (required-injection starting timing) where an injection starting delay period Tds elapses after the driving pulse is generated, and the injection rate starts decreasing at a point in time where a valve closing pressure reaching period Tde1 elapses after the driving pulse is terminated. Therefore, the geometric shape defined by the injection rate is represented as a reference triangle α shown in FIG. 9B, and an actual injection amount Q′ is given by an amount corresponding to the area of the reference triangle α (a required injection amount Q).
Generally, when the fuel pressure supplied to the injector is increased due to a pulsation, the injection starting delay period Tds becomes shorter as shown by an arrow (1) in FIG. 9B, and the injection starting timing becomes earlier. The maximum injection rate becomes larger as shown by an arrow (2) in FIG. 9B and a needle moving down period Tde2 becomes longer as shown by an arrow (3) in FIG. 9B. As a result, the geometric shape defined by the injection rate is represented by a large triangle β in FIG. 9B, and the actual injection amount Q′ is given by an amount corresponding to the area of the large triangle β, which is larger than the required injection amount Q.
On the other hand, when the injection starting delay period Tds is extended due to the pulsation, the injection starting timing is delayed. In this case, the geometric shape defined by the injection rate becomes smaller than the reference triangle α, and the actual injection amount Q′ becomes smaller than the required injection amount Q.
In a conventional method for evaluating an injector driving period Tqf, a base driving period is evaluated by use of a two-dimensional map of the injection rate and a common rail pressure (an example of a fuel supply pressure), and the base driving period is corrected by use of a two-dimensional correction map of the common rail pressure and an interval (a non-injection period from the total injection) provided for each injection period.
As mentioned above, the injector driving period Tqf has conventionally been determined through evaluating the base driving period by use of the two-dimensional map of the injection rate and the common rail pressure and correcting the base driving period by use of the two-dimensional correction map provided for each injection stage. Therefore, the number of the two-dimensional correction maps increases as the number of the injection stages increases, resulting in a disadvantageous increase in the man-hour for adaptation.
In addition, when only a part of the specifications of the injector is modified, the entire data for adaptation must be reevaluated. Therefore, a process for adaptation associated with the change in injector specifications becomes significantly complicated, causing the process for adaptation to be very inefficient. See, for example, Japanese Patent Laid-Open Publication No. Hei 10-266888.